Often, exercise machines, including those that have linkages that vary the resistance force applied to the user, provide a resistance force to the user's applied force, which resistant force is either linear or is not balanced against the effort force of the user. In some exercises, such as leg presses, the upper leg bone and the lower leg bone initially proscribe a fairly narrow angle, with the knee at the apex. During the leg press, the muscle operates to flex the lower leg away from the upper leg and to increase the angle between the two, at the knee. However, as the angle between the upper and lower leg at the knee changes, the user gets better leverage from the muscles involved and can apply greater effort force.
All machines would have balance between resistance force and user's effort force (by laws of physics), but not necessarily the user's maximum effort force. Applicant seeks to balance resistance to the strength of the user so as to effect optimal exercise over the entire range of motion. In the leg press, for example, the motion ranges from a position of high knee flexion (narrow angle between shank and thigh) to a position of full knee extension (zero knee flexion, corresponding to 180 degree angle between shank and thigh). It happens that the capacity of the user to generate force, due to muscle leverage, increases dramatically as the knee moves towards full extension.
It is often advantageous to have an exercise machine, including, for example, a leg press machine in which the resistance force (the force opposing the applied force that the exercise user places on the machine) also increases as, for example, the leg angle of the user increases.
It is advantageous, for the sake of optimizing exercise benefits, to provide a resistance which varies with the strength (i.e., the maximum effort capacity) of the joint as the joint position varies. For the leg press, this would mean that the machine resistance also increases as the leg position moves towards knee extension.